The planarity of wafer surfaces is very important when manufacturing integrated circuits. Photolithographic processes are typically pushed close to the limit of resolution in order to create maximum circuit density. The minimum critical dimensions which are typically required on a circuit are very small. Because these circuits are produced using photolithography it is essential that the wafer surface be highly planar in order that the electromagnetic radiation used to create a mask may be accurately focused at a single level, resulting in precise imaging over the entire surface of the wafer. If the wafer surface is not sufficiently planar the resulting mask will be poorly defined which may cause the circuit to malfunction.
Chemical mechanical planarization processes are used to achieve the degree of planarity required to produce ultra high density integrated circuits. Chemical mechanical planarization (CMP) processes involve planarizing a wafer by pressing it against a moving polishing surface that is wetted with a chemically reactive, abrasive slurry. The slurry is usually either basic or acidic and generally contains alumina or silica particles. The polishing surface is typically a planar pad made of a relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platen.
A conventional rotational CMP apparatus is illustrated in FIG. 1. A semiconductor wafer 112 is held by a wafer carrier 111. A soft, resilient pad 113 is positioned between the wafer carrier 111 and the wafer 112. The wafer 112 is held against the pad 113 by a partial vacuum. The wafer carrier 111 is continuously rotated by a drive motor 114 and is also designed for transverse movement as indicated by the arrow 115. The rotational and transverse movement is intended to reduce variability in material removal rates over the surface of the wafer 112. The apparatus further comprises a rotating platen 116 on which is mounted a polishing pad 117. The platen 116 is relatively large in comparison to the wafer 112, so that during the CMP process, the wafer 112 may be moved across the surface of the polishing pad 117 by the wafer carrier 111. A polishing slurry containing a chemically reactive solution, in which are suspended abrasive particles, is deposited through a supply tube 118 onto the surface of the polishing pad 117.
A top view of a typical polishing table of the prior art is illustrated in FIG. 2. The surface of the polishing table 1 is precision machined to be quite flat and may have a polishing pad affixed to it. The surface of the table rotates the polishing pad past one or more wafers 3 to be polished. The wafer is held by a wafer holder, as illustrated in FIG. 1, which exerts vertical pressure on the wafer against the polishing pad. The wafer holder may also rotate or orbit the wafer on the table during wafer polishing.
Alternatively, the table 1 may be stationary and serve as a supporting surface for individual polishing platens 2 each having their own individual polishing pad. As taught by U.S. Pat. No. 5,232,875, issued to Tuttle et al., each platen may have its own mechanism for rotating or orbiting the platen 2. A wafer holder will bring a wafer in contact with the platen 2 and an internal or external mechanism to the wafer holder may be used to also rotate the wafer during the polishing operation. In this polishing table, having multiple individual platens, each platen must be precision machined. While precision machining each individual platen is difficult, it is more difficult to precision machine a large moving table holding a polishing pad many times the area of an individual platen.
The wafers 3 are typically stored and transported in wafer cassettes which hold multiple wafers. The wafers 3 or wafer holders are transported between the wafer cassettes and the polishing table 1 using the wafer transport arm 4. The wafer transport arm 4 will transport the wafers 3 between the polishing table and the stations 5, which may be wafer cassette stations or wafer monitoring stations.
The polishing characteristics of the polishing pad will change over time as multiple wafers 3 are polished by the polishing pad. This glazing or changing of the polishing characteristics will effect the planarization of the surface of the wafers 3 if the pads are not periodically conditioned and unglazed. The pad conditioner 6 is used to periodically unglaze the surface of the polishing pad. The pad conditioner 6 has a range of motion which allows it to come in contact with the individual pads and conduct the periodic unglazing and then to move back to its rest position, out of the way of the table, during the polishing of the wafers.
As illustrated in FIG. 2, the table 1 may be used to simultaneously polish multiple wafers on its horizontal surface. Wafers arranged on the horizontal surface may be in a circular configuration, as shown, or they may be in a regular two-dimensional array such as 2.times.1, 2.times.2 or 3.times.2. The distribution of polishing locations on the table 1 in the horizontal xy dimension requires a complex combination of table motion and/or pick and place robotic mechanisms so that the transport arm 4 is able to transport the wafers 3 from the start to finish of the polishing operations and the polishing pad 6 is able to perform the pad conditioner operations and then to retreat to its rest position.
U.S. Pat. No. 5,232,875 to Tuttle et al. teaches that the pressure between the surface of the wafer to be polished and the moving polishing pad may be generated by either the force of gravity acting on the wafer and the wafer carrier or by mechanical force applied normal to the wafer surface. Tuttle et al. also teaches that the slurry may be injected through the polishing pad onto its surface. The planar platens taught by Tuttle et al. are moved in a plane parallel to the pad surface with either an orbital, fixed-direction vibratory, or random direction vibratory motion.
The horizontal polishing tables as taught by the prior art take up a large amount of valuable floor space within the manufacturing facility. What is needed is a wafer polishing apparatus which achieves more wafer throughput within the same amount of floor space thereby minimizing the floor space required within the manufacturing facility per polished wafer. What is further needed is a wafer polishing apparatus which does not require the complex robotic systems, capable of movement within the xy direction, as necessary for polishing tables of the prior art to transport wafers to and from a horizontal polishing table and also to polish the platens on a horizontal polishing table.